The situation is not quite as straight forward as you seem to believe: https://en.wikipedia.org/wiki/Marsh_v._Alabama

> "Ownership does not always mean absolute dominion. The more an owner, for his advantage, opens up his property for use by the public in general, the more do his rights become circumscribed by the statutory and constitutional rights of those who use it."

Marsh v Alabama may not currently apply to digital spaces, but it's not inconceivable that in the future there will be another SCOTUS ruling to clarify this new emerging scenario.

Sure it does. FB is a communication platform that adheres to the principles of free speech or it isn't.

Here, they are demonstrating (among other times) that they do not believe in free speech. It is their right. Just as it is their right to collect user data and resell it.

They have a right to suck and be evil.

Facebook aren't hosting it.